1. Field of the Invention
This invention relates generally to the field of medical monitoring devices, and more particularly relates to medical devices used to detect low amplitude artifacts produced by artificial cardiac pacing.
2. Description of the Prior Art
A wide variety of cardiac pacemakers are known and commercially available. Pacemakers are generally characterized by which chambers of the heart they are capable of sensing, the chambers to which they deliver pacing stimuli, and their responses, if any, to sensed intrinsic electrical cardiac activity. Some pacemakers deliver pacing stimuli at fixed, regular intervals without regard to naturally occurring cardiac activity. More commonly, however, pacemakers sense electrical cardiac activity in one or both of the chambers of the heart, and inhibit or trigger delivery of pacing stimuli to the heart based on the occurrence and recognition of sensed intrinsic electrical events. A so-called "SSI" pacemaker, for example, senses electrical cardiac activity in a chamber, atrium (A) or ventricle (V), of the patient's heart, and delivers pacing stimuli to the same chamber only in the absence of electrical signals indicative of natural chamber contractions. A "DDD" pacemaker, on the other hand, senses electrical signals in both the atrium and ventricle of the patient's heart, and delivers atrial pacing stimuli in the absence of signals indicative of natural atrial contractions, and ventricular pacing stimuli in the absence of signals indicative of natural ventricular contractions. The delivery of each pacing stimulus by a DDD pacemaker is synchronized with prior sensed or paced events.
Pacemakers are also known which respond to other types of physiological based signals, such as signals from sensors for measuring the pressure inside the patient's ventricle or measuring the level of the patient's physical activity. These devices are labeled "SSIR" for a single chamber version or "DDDR" for a dual chamber version.
As pacemaker technology as well as integrated circuit and lead technologies have evolved, chronic pacing thresholds have approached very low values (&lt;&lt;1.0 volts) and very efficient low current CMOS amplifier designs have become available. Instrumentation OP Amps for acquiring biopotential signals (ECG, EEG, EMG, etc.) as well as electrical signals produced by implantable devices such as cardiac pacemakers are now common in the art. Along with the development of such instrumentation OP Amps, the concepts associated with using a driven right leg system for canceling out 60 HZ common mode noise and noise from florescent lights have evolved.
U.S. Pat. No. 5,161,529, issued to Stotts et al. describes a circuit in which the frequency response of the sense amplifier is selectively set to detect cardiac activity in the form of the intrinsic QRS pattern, but which is switched to a lower frequency bandpass to render it more responsive to an evoked potential at the moment that a stimulating pulse is delivered to the heart.
U.S. Pat. No. 4,226,245, issued to Bennett, Jr. describes circuitry which is capable of distinguishing a pacing pulse from a patient's QRS wave in the presence of noise sources such as 60 HZ line noise or other noise sources having signal rise times in the millisecond range.
U.S. Pat. No. 3,946,744, issued to Auerbach describes circuitry utilizing a driven right leg electrode to eliminate common-mode noise generated by 60 Hz power line frequency as well as other pickup noise signals in a system which transmits pacing artifact information.
At times, however, the signal-to-noise ratio of electrical signals discussed hereinbefore, becomes too low for the prior art systems to adequately identify the high frequency pacing artifact, particularly when the pacing artifact's frequency spectrum lies near the middle of the spectrum of noise generated by multiple noise generators. What is needed is a system or device capable of canceling out high frequency noise caused by a multitude of noise sources, including but not limited to switched mode power supplies, DC/DC converters, and the noise generated by a pacemaker programming head, in addition to those noise sources discussed hereinbefore.